This invention relates to a power supply circuit, and more particularly, but not exclusively, for powering a fluorescent tube.
Compact fluorescent lamps have recently become popular, and are sold, for example, by Philips under their Energy Saver brand. Such lamps have a much higher efficiency than corresponding incandescent lamps and also have a substantially longer life.
A fluorescent tube in such a design is conventionally driven by a switching power supply, which can be generally represented as load 10 in FIG. 1, in which load 10 is placed in parallel with an electrolytic capacitor 20, the load 10 and capacitor 20 that is connected to a main voltage supply 30 via a rectifier 40. For such a circuit, it is essential to provide the electrolytic capacitor 20 for smoothing the rectified current and as a d.c. energy reservoir, in order to provide a high-demand-working current to the load. It is a disadvantage of such circuit that the capacitor 20 acts as a peak detector and displays a serious capacitative effect. As shown with reference to FIG. 2, the input current from the main voltage supply, Imain, charges the electrolytic capacitor 20 only when the instantaneous AC voltage exceeds the voltage across the electrolytic capacitor. Thus, the waveform of the main current Imain does not follow a sinusoidal pattern. By comparison of the main voltage supply, Vmain, with the current Imain it can be seen that the fluorescent lamp circuit of FIG. 1 has a very poor power factor.
Having a low power factor has several disadvantages. In particular that, larger components are required in the power supply circuit for the same light output or, alternatively, existing components have to work harder, thus increasing heat dissipation. Furthermore, in commercial establishments such as hotels and factories, where there is a three phase supply, it is necessary to maintain a good power factor over all three phases. Otherwise, there is a risk of unbalanced line power. For this reason, particularly, USA and EEC Regulatory Authorities insist on a power factor of greater than 0.85. In general, a circuit of the kind shown in FIG. 1 will have a power factor of 0.4-0.6.
Accordingly, it is an object of the present invention to provide a power supply circuit having an improved power factor.